[PATCH 1/2] KVM: x86: Select lowest vCPU ID for aliased xAPIC IDs

[Partha Sarathi Satapathy <partha.satapathy@oracle.com>]

From: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>

When KVM_X2APIC_API_USE_32BIT_IDS is enabled, KVM's x2APIC hotplug
quirk allows events to target a vCPU by its x2APIC ID while the vCPU
is still in xAPIC mode.

In xAPIC mode, APIC IDs are truncated to eight bits. Consequently,
vCPUs with distinct x2APIC IDs can share an effective physical APIC ID.
KVM's x2APIC erratum specifies that an event targeting an aliased
physical APIC ID is delivered only to the vCPU with the lowest vCPU ID.

However, the optimized physical APIC map does not guarantee lowest-ID
selection. When rebuilding the map, KVM retains the first xAPIC-mode
vCPU encountered for an aliased 8-bit xAPIC ID. Since vCPUs can be
added in any order, that vCPU need not have the lowest vCPU ID.

For example, vCPU 19 has x2APIC ID 0x013 and vCPU 275 has x2APIC ID
0x113. Both expose xAPIC ID 0x13 while in xAPIC mode. If vCPU 275 is
created before vCPU 19, the map retains:

  phys_map[0x13] = vCPU 275

An INIT/SIPI targeting physical destination 0x13 then starts vCPU 275
instead of vCPU 19. vCPU 275 executes the secondary startup path for
the firmware CPU entry with APIC ID 0x013. After vCPU 275 enables
x2APIC, Linux observes APIC ID 0x113 and reports:

  Firmware: 13 APIC: 113

KVM hardcodes a vCPU's x2APIC ID to its vCPU ID. When populating an
aliased legacy xAPIC entry, retain the vCPU with the lowest vCPU ID.
This makes optimized-map selection independent of vCPU-add order. The
separate full-ID entry remains unaffected, and destination 0x113
continues to select vCPU 275.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Signed-off-by: Partha Sarathi Satapathy <partha.satapathy@oracle.com>
---
 arch/x86/kvm/lapic.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 1597dd0b0cc6..5dedffddebec 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -245,7 +245,9 @@ static int kvm_recalculate_phys_map(struct kvm_apic_map *new,
 		if (apic_x2apic_mode(apic) || x2apic_id > 0xff)
 			new->phys_map[x2apic_id] = apic;
 
-		if (!apic_x2apic_mode(apic) && !new->phys_map[xapic_id])
+		if (!apic_x2apic_mode(apic) &&
+		    (!new->phys_map[xapic_id] ||
+		     x2apic_id < new->phys_map[xapic_id]->vcpu->vcpu_id))
 			new->phys_map[xapic_id] = apic;
 	} else {
 		/*
-- 
2.43.7